1. (Field of the Invention)
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system having air-fuel ratio sensors arranged, respectively upstream and downstream of a three-way catalyst mounted across an exhaust passage of the engine.
2. (Prior Art)
Conventionally, there has been widely used a method of feedback-controlling the air-fuel ratio of a mixture supplied to an internal combustion engine (hereinafter referred to as the "supply air-fuel ratio") based on results of comparison between an output from an air-fuel ratio sensor arranged upstream of a three-way catalyst mounted across an exhaust passage of the engine and a reference value. Further, control systems are also conventionally known in which the feedback control is corrected based on an output from a second air-fuel ratio sensor additionally provided downstream of the three-way catalyst. Such control systems include the following ones:
(1) An air-fuel ratio control system proposed by Japanese Provisional Patent Publication (Kokai) No. 53-103796, in which the reference value used in the feedback control is changed according to the output from the second sensor in order to determine whether the performance of the first sensor has been degraded.
(2) Air-fuel ratio control systems proposed by Japanese Provisional Patent Publications (Kokai) Nos. 61-234241 and 61-232349, in which a proportional term or an integral term used in the feedback control are set according to the output from the second sensor in order to improve the responsiveness of the feedback control when the performance of the first sensor is degraded.
In general, when the engine operating condition has shifted to a high load region, the air-fuel ratio detected by the second sensor tends to be drastically enriched due to transient fuel-increasing correction. As a result, according to the air-fuel ratio control method employed by the above example (1), the reference value used in the feedback control is largely changed in the direction of leaning the air-fuel ratio, in response to the enriched second air-fuel ratio. Therefore, the supply air-fuel ratio is more often subjected to leaning correction, which makes it impossible to readily obtain a rich air-fuel mixture for achieving the required high engine output. This unfavorable state will then be eliminated by a gradual increase in the reference value through feedback control. However, there is still the problem that sufficient engine output cannot be obtained immediately after transition of the engine operating condition to the high load region, resulting in degraded driveability.